There are many disease states that cause bone defects in the spinal column. For instance, osteoporosis and other metabolic bone conditions weaken the bone structure and predispose the bone to fracture. If not treated, certain fractures and bone defects of the vertebral body may produce intolerable pain, and may lead to the development of deformity and severe medical complications.
Bone weakening may also result from benign or malignant lesions of the spinal column. Tumors often compromise the structural integrity of the bone and thus require surgical stabilization and repair of defects with biocompatible materials such as bone grafts or cements. Bone tumors of the spine are relatively common, and many cause vertebral compression fracture.
More than 700,000 osteoporotic compression fractures of the vertebrae occur each year in the United States—primarily in the elderly female population. Until recently, treatment of such fractures was limited to conservative, non-operative therapies such as bed rest, bracing, and medications.
One surgical technique for treating vertebral compression fracture can include injecting or filling the fracture bone or bone defect with biocompatible bone cement. A relatively new procedure known as “vertebroplasty” was developed in the mid 1980's to address the inadequacy of conservative treatment for vertebral body fracture. This procedure involves injecting radio-opaque bone cement directly into a fracture void, through a minimally invasive cannula or needle, under fluoroscopic control. The cement is pressurized by a syringe or similar plunger mechanism, thus causing the cement to fill the void and penetrate the interstices of a broken trabecular bone. Once cured, the cement stabilizes the fracture and eliminates or reduces pain. Bone cements are generally formulations of non-resorbable biocompatible polymers such as PMMA (polymethylmethacrylate), or resorbable calcium phosphate cements which allow for the gradual replacement of the cement with living bone. Both types of bone cements have been used successfully in the treatment of bone defects secondary to compression fractures of the vertebral body.
One technique which has gained popularity in recent years is a modified vertebroplasty technique in which a “balloon tamp” is inserted into the vertebral body via a cannula approach to expand or distract the fractured bone and create a void within the cancellous structure. Balloon tamps are inflated using pressurized fluid such as saline solution. The inflation of a balloon membrane within the bone produces radial expansion forces on the surface of the membrane and forms a cavity in the bone. When deflated and removed, the membrane leaves a cavity that is subsequently filled with bone cement. The formation of a cavity within the bone allows for the injection of more viscous cement material, which may be relatively less prone to leakage.
In certain instances, such as the treatment of acute or mobile fractures, the balloon is also effective at “reducing” the fracture and restoring anatomic shape to a fractured body. In particular, balloon dilatation in bone is maximally effective if the balloon device is targeted inferior to, or below, the fracture plane. In this instance, the balloon dilatation may distract, or lift, a fracture bone fragment, such as the vertebral body endplate.
One limitation to the use of such balloon dilatation has been the difficulty in effectively targeting the location within the bone at which the cavity should be created prior to dilatation of the balloon. In the specific case of vertebral body fracture, there are anatomical challenges to targeting with minimally invasive instrumentation. Safe passage of instruments and balloon catheters from the posterior surgical approach is generally achieved through a straight cannula positioned within the pedicle of the vertebral body, or just lateral to the pedicle to avoid potentially dangerous penetration of the cannula in the spinal canal. This anatomically defined trajectory often does not align with, or target, the fracture within the vertebral body. This limits the effectiveness of such techniques in effectively targeting a fracture.